The present invention relates to a communication terminal for exercising transmission power control, which is employed, for instance, for a CDMA (Code Division Multiple Access) communication system.
In the CDMA communication system, for example, adaptive transmission power control is exercised to minimize the transmission power during a signal transmission/reception between a cellular phone terminal and a base station and maintain the strength of a radio wave received by the base station at a certain level. Particularly, recently released cellular phone terminals incorporate an APC (Auto Power Control) circuit in order to provide transmission power control for the CDMA communication system with high accuracy. The APC circuit receives a transmission output from a transmission power amplifier, converts the received transmission output to a voltage value (detection output value) with a wave detector, and exercises control to ensure that the transmission power coincides with a desired expected value. More specifically, if the detection output value is smaller than the expected value of the transmission power, the APC circuit exercises control so as to increase a transmission power value. If, on the contrary, the detection output value is greater than the expected value of the transmission power, the APC circuit exercises control so as to decrease the transmission power value.
The technology disclosed by Japanese Patent Laid-open No. Hei 10-173548 (Patent Document 1) defines the relationship between the AGC (Automatic Gain Control) voltage of a transistor IC in a transmission circuit and the detection output for APC control prior, for instance, to shipment, determines a threshold value for the AGC control voltage and a threshold value for the detection output for APC control in accordance with the defined relationship, stores the threshold values in memory, and notes the detection output value, AGC control voltage value, and threshold values stored in memory during an actual communication to judge whether APC control is normally exercised. If APC control is abnormally exercised, the disclosed technology stops a transmission process to prevent the circuit from being rendered faulty or damaged.
It is known that communication failure or circuit destruction may occur if, for instance, the detection output value is abnormal while the above APC control is exercised. More specifically, when the detection output value remains, for instance, small due to an amplifier failure or other circuit abnormality, the above APC control is exercised to continuously increase the transmission power. An excessive input then enters the transmission power amplifier so that in the worst case, the circuit would become damaged. If, on the contrary, the detection output value remains great due to circuit board short-circuiting or other circuit abnormality, the above APC control is exercised to continuously decrease the transmission power. This eventually incurs a communication breakdown.
The technology disclosed by Patent Document 1 can check for an abnormal APC control operation in accordance with the AGC control voltage value and detection output value for actual communication and the threshold values stored in memory. If any abnormal APC control operation is recognized, the disclosed technology can stop a transmission process to prevent the circuit from becoming faulty or damaged.
However, the technology disclosed by Patent Document 1 has to define the relationship between the AGC control voltage and the detection output for APC control on an individual cellular phone terminal basis prior, for instance, to shipment, and determine a threshold value for the AGC control voltage and a threshold value for the detection output for APC control in accordance with the defined relationship. It means that the threshold values vary with the circuit characteristics of each cellular phone terminal. It is therefore necessary to determine the threshold values for each cellular phone terminal and store the determined threshold values in memory or otherwise prior to shipment. The technology disclosed by Patent Document 1 is not favorable because it reduces productivity significantly and incurs a production cost increase.